In modern practice the most popular medical cryoprobes make use of gaseous or liquid refrigerants (carbon dioxide, freons, liquid nitrogen and others) which can be divided into two basic types, according to the method of refrigerant supply, namely cryoprobes with discrete and continuous types of supply.
Cryoprobes of the first type are basically reservoirs filled with liquid refrigerants in advance to cool a metal rod. The latter is in fact the probe. The temperature of the cryoprobe tip cannot, therefore, be controlled in any way in the course of an operation and is relatively low (about -40.degree. C.).
The probe can be warmed up in case of a dangerous development in the operation (deep freezing, freezing of surrounding tissue, etc.) by means of a warm sterile liquid fed directly to the freezing area.
The process of warming up is lengthy (tens of seconds) and reapplication of the cryoprobe in the operation required refilling of the refrigerant.
The above mentioned unfavourable developments are even more frequent since the cryoprobe is introduced into the operation field already cooled, as well as due to the fact that the probe tip has cooled side surfaces.
Cryoprobes using continuously supplied refrigerant are much more efficient, but, consequently, more complicated. In this case use is made of complex gas apparatuses, maintenance becomes quite a problem, as well as preparation of the cryoprobe for an operation. A great reserve of refrigerant is a necessity.
A new medically oriented cryoprobe based on the Peltier effect method of thermoelectric cooling was proposed in the 60's (cf., for example, I. K. Poltinnikova, E. A. Kolenko, Intracapsular Extraction of a Cataract by Semiconductor Device. Ophthalmotological Magazine, No. 8, 1964, pp.563-566. in Russian).
The medical thermoelectric cryoprobe comprises a semiconductor cooling thermoelement and a double handle. The p and n branches of the thermoelement are mounted on the two halves of the handle.
Refrigerating power of the thermoelement is concentrated on the functional surface of the cryoprobe which is in fact a metal concentrator-tip placed on the switching plate of the thermoelement. The specific refrigeration effect of the functional cryoprobe surface required for freezing is attained in this manner.
Heat sink from the hot junctions of such thermoelements in cryoprobes is effected with the help of running water circulating inside the handle. The masive concentrator-tip determines in this case great thermal inertia of the cryoprobe. For cooling such a cryoprobe should be cut in 3 minutes before freezing the tissue. It is, therefore, still necessary to introduce a pre-cooled probe into the operation field, its side cooled surfaces being large as compared to the functional working surface.
As it has already been mentioned these features are liable to produce dangerous developments in operations, particularly in ophthalmological operations where the field of operation is very small.
Moreover, in such conditions water cooling hoses feeding water to the cryoprobe handle make it very hard to manipulate the cryoprobe.